1. Field of the Invention
The present invention relates to an imaging apparatus having an image sensor and to a control method therefor.
2. Description of the Related Art
An image sensor generates dark current in the case of performing long-time exposure with the image sensor, in the case of shooting at a high International Organization for Standardization (ISO) sensitivity, or in the case of shooting at a high temperature.
Dark current is generated because a solid-state image sensor has a characteristic of not only converting light energy into an electrical signal but also converting thermal energy into an electrical signal. Dark current has a high temperature dependency. When the temperature rises by 80 to 10° C., the amount of dark current output becomes twice as large.
In a solid-state image sensor, dark current affects image quality. Accordingly, manufacturers of an image sensor have used various methods for preventing high dark current from occurring.
When the temperature of the entire image sensor is high due to the environment under which the image sensor exists and a part of the image sensor (for example, an output stage amplifier) consumes much current (power), the temperature of the image sensor can partially rise. In such a case, dark current increases in the portion whose temperature has risen and the total amount of dark current generated in the image sensor becomes large.
For example, in the case of shooting a nightscape in a long exposure time (at a high ISO sensitivity), light of a magenta-like color appears in an area of an image when no light actually exists in the nightscape.
When the level of dark current in an image sensor becomes high and when dark current varies with pixel areas, the luminance and color balance in the image sensor can be affected. When the level of dark current becomes high in the entire image sensor, a dark level increases and, thus, the dynamic range of a photoelectrical signal decreases. Thus, an increase in dark current significantly affects image quality.
As a method for eliminating or reducing such affection from dark current, noise reduction processing (dark image subtraction processing) in a long exposure time is known. In the noise reduction processing, after an original image is taken, an image (a dark image) is taken in a state in which an image sensor is light-shielded under the same condition as the condition for shooting the original image, and then the taken dark image is subtracted from the original image.
In this regard, the dark image subtraction processing has the following problems. Firstly, in correctly and precisely eliminating the affection from dark current generated at the time of shooting an original image, it is desirable to obtain a dark image under the same condition as the condition for shooting the original image. Accordingly, when a long-time exposure is performed, a time as long as the time for the long-time exposure is additionally required for obtaining a dark image. Thus, the operability considerably degrades.
Secondly, in performing processing for subtracting a dark image from an original image, the level of random noise in the original image and the dark image increases to a √2 (root 2) multiple of that of the original image. Thus, a degradation of image quality cannot essentially be prevented.
In this regard, Japanese Patent Application Laid-Open No. 2005-142829 discusses a method in which a dark current component at the time of shooting an original image is detected and noise reduction processing is performed only when a resulting value exceeds a predetermined value, in order to possibly avoid performing noise reduction processing when the shooting condition does not necessarily require noise reduction processing.
Meanwhile, a white flaw, i.e., one of pixel defects of an image sensor, occurs in some cases. A white flaw is a pixel that appears as a white spot in an image, which occurs when the dark current in the pixel is considerably high compared to the dark current in other pixels and the output of the dark current thus becomes very high due to temperature rise or long exposure time.
Dark current at a white flaw is generated due to a crystal defect in a photodiode portion (a main portion constituting pixels of an image sensor) and changes according to exposure time and the temperature of the image sensor. Accordingly, a white flaw greatly affects dark current under a high temperature environment and in the case of long-time exposure. Furthermore, in the case where a high ISO sensitivity is used, that is, when a high level of gain can be applied to an output from the image sensor, an actual image is affected from a white flaw by the level equivalent to the level of the gain.
As a characteristic of a white flaw occurring dependent on temperature and exposure time, white flaws are distributed in several different slopes in a graph illustrating a relationship between exposure time and the output of white flaws under a predetermined temperature.
In determining whether to perform noise reduction processing, it is necessary to satisfy the following two requisites. First, noise reduction processing should not be performed when unnecessary in order to avoid an affection from noise reduction. Second, since the determination as to whether to perform noise reduction processing is automatically made in a conventional method, if any affection from dark current occurs, noise reduction processing is immediately performed. Thus, it is necessary to detect an amount of dark current with high accuracy.
However, it is very difficult to correctly and precisely determine an amount of dark current in an image area whose affection from dark current is barely visible. In particular, when an image is taken at a high ISO sensitivity and in a long exposure time, the image has a large amount of random noise components. Accordingly, in order to detect a correct absolute value of dark current under such condition, it is necessary to perform the detection after removing the random noise components.
Thus, in the case of processing with firmware, detection of an amount of dark current should be performed after reading a part of an image in an optically light-shielded area of an image sensor and removing random noise components. Thus, a large capacity of memory resources and a long time are required for calculation. In addition, it is difficult to secure a sufficient degree of detection accuracy.
In addition, in a system utilizing an analog front end (AFE) for clamping a dark level, a dark current component rarely appears on image data. Accordingly, it is difficult to perform a detection of dark current.
Furthermore, in a system utilizing an AFE for clamping a dark level, it is necessary to read a dark output before performing clamping of a dark level in order to detect an amount of dark current from a vertical optical black (VOB) portion. Moreover, in this regard, timing for starting a clamping operation within a limited area of an optical black portion needs to be delayed. Thus, lead-in time for clamping a large amount of dark current on an analog base is required. Accordingly, in a system utilizing an AFE, it is difficult to detect a dark current amount in a VOB portion.
In addition, the number of pixels whose outputs can be read before clamping is limited. Thus, it is very difficult to detect a precise dark current amount while eliminating an affection from noise in this case.
Meanwhile, the level of an output from a white flaw is higher than that of a dark current component from a normal pixel. Accordingly, if positional information of a minute defect pixel is previously available, an output of gain higher than dark current can be obtained by monitoring the pixel whose output is higher than the output of a dark current component from a normal pixel at the time of shooting. That is, by previously detecting a difference between a slope of dark current component distribution of a normal pixel and that of a white flaw, detection of a dark current component can be performed with high accuracy.
However, in order to perform such processing, it is necessary, during manufacturing of cameras, to detect, for each camera, a level and an address of a defect pixel and to store the detected address in each camera. In addition, in order to detect the level of output of a white flaw with high accuracy, it is necessary to detect a plurality of addresses of white flaws and to store the detected addresses in each camera. Such processing is very difficult to perform.